1. Field of the Invention
The present invention relates to a driving method of external electrode fluorescent lamp inverter for backlight, and more specifically, to a driving method of external electrode fluorescent lamp inverter in which a full-bridge inverter is used to drive an external electrode fluorescent lamp in a self-discharge driving manner, which is different from a cold cathode fluorescent lamp to be driven only by a sine wave, to thereby obtain high efficiency and high luminance.
2. Description of the Related Art
Recently, requests for a display device displaying various information such as video data, graphic data, or text data rapidly increase, in order to deliver a plenty of information in an information-oriented society which rapidly develops. In accordance with the requests, display industries are rapidly growing.
A thin-film-transistor liquid crystal display (TFT-LCD) is a high-tech display element for the next generation, which has lower power consumption than a cathode ray tube (CRT), can be thinned with a light weight, and does not give off harmful electromagnetic waves. The TFT-LCD has made remarkable progress for several years, which is used in various fields as well as in a personal computer. Recently, in the digital broadcasting age, a thin-film-transistor liquid crystal display (TFT-LCD) as well as a plasma display panel (PDP) as a large screen display (more than 30 inch) is getting attention, so worldwide foremost companies put their heart and soul into developing a large screen LCD.
The TFT-LCD is not a self-emission element, different from the PDP or CRT. Therefore, the TFT-LCD is composed of a back light unit (BLU) which is used as a back light source in the rear side of the TFT-LCD, a TFT array which delivers and controls electric signals, liquid crystal which determines transmission of light by changing the molecular structure according to an applied voltage, and a color filter.
Most of existing backlight units (BLU) use a cool cathode fluorescent lamp. The cool cathode fluorescent lamp is a special fluorescent lamp using the cathode which does not generate heat, which is suitable for characteristics of the TFT-LCD. While the cool cathode fluorescent lamp has low power consumption, it exhibits high luminance and is slim and long, so it is mostly used as a light source of LCD.
The cool cathode fluorescent lamp, filled with a certain amount of mercury and a mixed gas of argon (Ar) and neon (Ne), is provided with a glass tube of which the inner surface is coated with a fluorescent substance and internal electrodes which are installed in both ends of the glass tube.
As a method of driving the cool cathode fluorescent lamp, a sine wave driving method is widely used. In order to drive a sine wave, a resonant inverter is used. As a widely-used existing inverter, there is provided a push-pull inverter, a half-bridge inverter, or the like. In the case of the push-pull inverter and half-bridge inverter, a frequency modulation method is used or an inverter is constructed in two stages in order to control a voltage or current which is applied to a lamp.
Recently, another method is also used, which changes a duty ratio (a ratio of on to off of a switching element) by using a full-bridge inverter so as to control a voltage or current which is applied to the cool cathode fluorescent lamp.
While the cool cathode fluorescent lamp has internal electrodes, a recently-developed external electrode fluorescent lamp has electrodes formed outside. The external electrode fluorescent lamp is getting an attention as a light source for a large-screen LCD backlight unit. The external electrode fluorescent lamp having electrodes formed outside the tube has longer life span than the cool cathode fluorescent lamp. Further, since the plurality (about 20) of lamps can be driven in parallel and there is no voltage deviation between the lamps, it is possible to implement uniform luminance.
In the external electrode fluorescent lamp, several external electrode fluorescent lamps are driven by one inverter. Therefore, a full-bridge inverter is frequently used, of which a driving voltage is high and which can control a voltage and current by changing a duty ratio.
As described above, the external electrode fluorescent lamp has similar characteristics to the cool cathode fluorescent lamp and has capacitive load characteristic. Therefore, in the case of using a transformer, the external electrode fluorescent lamp resonates with the leakage inductance of the transformer. Such a characteristic is similar to that of the existing series resonant transformer.
Hereinafter, a method of driving the cool cathode fluorescent lamp inverter according to the related art will be described with reference to the accompanying drawings.
FIG. 1 is a circuit diagram of a general cool cathode fluorescent lamp inverter for backlight, showing a full-bridge inverter circuit.
As shown in FIG. 1, the full-bridge inverter is composed of first to fourth switching elements M1 to M4 between a DC power supply Vdc and ground. Switching operations of the first to fourth switching elements M1 to M4 allow a voltage to be applied to the primary side of a transformer through a first output terminal A connected between the first and second switching elements M1 and M2 and through a second output terminal B connected between the third and fourth switching elements M3 and M4.
The first switching element M1 composed of an NMOS transistor or a PMOS transistor (the PMOS transistor is shown in FIG. 1) is turned on by a first control signal S1, which is applied to a gate, so as to serve to switch the DC power supply Vdc to the first output terminal A to pull up a voltage level of the first output terminal A. The first switching element is turned on when the signal S1 is ‘low’. On the contrary, the second switching element M2 composed of an NMOS transistor is turned on by a second control signal S2, which is applied to a gate, so as to serve to pull down the voltage level of the first output terminal A to a ground voltage level Vss. The second switching element is turned on when the signal S12 is ‘high’.
As in the first switching element M1, the third switching element M3 composed of an NMOS transistor or PMOS transistor (the PMOS transistor is shown in FIG. 1) is turned on by a third control signal S3, which is applied to a gate, so as to serve to switch the DC power supply Vdc to the second output terminal B to pull up a voltage level of the second output terminal B. The third switching element is turned on when the signal S3 is ‘low’. On the contrary, the fourth switching element M14 composed of an NMOS transistor is turned on by a fourth control signal S4, which is applied to a gate, so as to serve to pull down the voltage level of the second output terminal B to the ground voltage level Vss. The fourth switching element M4 is turned on when the signal S4 is ‘high’.
The voltage, applied to both ends of the primary side of the transformer through the first and second output terminals A and B, is the DC power supply voltage Vdc applied from the full-bridge inverter. If the voltage is applied to the primary side of the transformer, a voltage amplified in the secondary side of the transformer drives a cool cathode fluorescent lamp.
FIG. 2 is a diagram showing main waveforms when the cool cathode fluorescent lamp inverter according to the related art operates.
As shown in FIG. 2, an operational process of the cool cathode electrode fluorescent lamp inverter will be described as follows, in which one period Ts of waveform is divided into eight intervals {circle around (1)} to {circle around (8)}.
First, in the first interval {circle around (1)}, the first switching element M1 which is an PMOS transistor is turned off because the first control signal S1 is ‘logic high’, the second switching element M2 which is an NMOS transistor is turned off because the second control signal S2 is ‘logic low’, and the third switching element M3 which is an PMOS transistor is turned off because the third control signal S3 is ‘logic high’. On the contrary, the fourth switching element M14 which is an NMOS transistor is turned on because the fourth control signal S14 is ‘logic high’.
In the first interval {circle around (1)}, the first to third switching element M1 to M3 are turned off and only the fourth switching element M4 is turned on. Therefore, the power supply voltage Vdc is applied to the primary side of the transformer, because the electric current in the primary side of the transformer flows through the body diode of the switching element M1.
In the second interval {circle around (2)}, the first switching element M1 is turned on because the first control signal S1 changes from ‘logic high’ to ‘logic low’, and the second switching element M2 is continuously turned off because the second control signal S2 is continuously ‘logic low’. Further, the third switching element M3 is continuously turned off because the third control signal S3 is continuously ‘logic high’, and the fourth switching element M4 is continuously turned on because the fourth control signal S4 is continuously ‘logic high’.
Therefore, in the second interval {circle around (2)}, the power supply voltage Vdc is applied to the primary side of the transformer through the first switching element M1 because the first and fourth switching elements M1 and M4 are turned on. At this time, the electric current Ipri flowing in the primary side of the transformer slowly increases and then decreases, as shown in FIG. 2. That is because the leakage inductance Lr of the transformer resonates with the capacitance Cr of the lamp.
In the third interval {circle around (3)}, the first switching element M1 is continuously turned on because the first control signal S1 is continuously ‘logic low’, and the second switching element M2 is continuously turned off because the second control signal S2 is continuously ‘logic low’. Further, the third switching element M3 is continuously turned off because the third control signal S3 is continuously ‘logic high’, and the fourth switching element M4 is turned off because the fourth control signal S4 changes from ‘logic high’ to ‘logic low’.
Since the first switching element M1 is turned on and the second to fourth switching elements M2 to M4 are turned off in the third interval {circle around (3)}, a voltage of 0 V is applied to the primary side.
The moment the fourth switching element M4 is turned off, a voltage of 0 V is applied to the primary side of the transformer while the output capacitor of the fourth switching element M4 is charged and the output capacitor of the third switching element M3 is discharged. If the output capacitor of the third switching element M3 is completely discharged, the electric current Ipri flows through the body diode of the second and third switching elements M2 to M3 to thereby set up a zero voltage switching condition.
In the fourth interval {circle around (4)}, the first switching element M1 is continuously turned on because the first control signal S1 is continuously ‘logic low’, and the second switching element M2 is continuously turned off because the second control signal S2 is continuously ‘logic low’. Further, the third switching element M3 is turned on because the third control signal S3 changes from ‘logic high’ to ‘logic low’, and the fourth switching element M4 is continuously turned off because the fourth control signal S14 is continuously ‘logic low’.
Since the first and third switching element M1 and M3 are turned on and the second and fourth switching elements M2 and M4 are turned off in the fourth interval {circle around (4)}, a voltage to be applied to the primary side of the transformer is 0 V as it is. Therefore, the direction of current flowing in the primary side of the transformer does not change.
In the fifth interval {circle around (5)}, the first switching element M1 is turned off because the first control signal S1 changes from ‘logic low’ to ‘logic high’, and the second switching element M2 is continuously turned off because the second control signal S2 is continuously ‘logic low’. Further, the third switching element M3 is continuously turned on because the third control signal S3 is continuously ‘logic low’, and the fourth switching element M4 is continuously turned off because the fourth control signal S4 is continuously ‘logic low’.
Therefore, in the fifth interval {circle around (5)}, the first, second, and fourth switching elements M1, M2, and M4 are turned off, and only the third switching element M3 is turned on. At this time, the direction of current flowing in the primary side of the transformer in the fourth interval {circle around (4)} has not changed. Therefore, if the first switching element M1 is turned off, the power supply voltage Vdc is supplied to the minus (−) terminal of the primary side of the transformer through the third switching element M3 while the electric current flows through the body diode of the second switching element M2. Therefore, as shown in FIG. 2, the voltage Vpri of the primary side of the transformer whose voltage level has been 0 V descends to the negative (−) potential when the first switching element M1 is turned off. At this time, the electric current flowing in the primary side of the transformer flows in the reverse direction through the body diode of the second switching element M2.
In addition, as the voltage is applied, the voltage polarities of both electrodes of the cool cathode fluorescent lamp change in the fifth interval {circle around (5)}.
In the sixth interval {circle around (6)}, the first switching element M1 is continuously turned off because the first control signal S1 is continuously ‘logic high’, and the second switching element M2 is turned on because the second control signal S2 changes from ‘logic low’ to ‘logic high’. Further, the third switching element M3 is continuously turned on because the third control signal S3 is continuously ‘logic low’, and the fourth switching element M4 is continuously turned off because the fourth control signal S4 is continuously ‘logic low’.
In the sixth interval {circle around (6)}, the second and third switching element M2 and M3 are turned on, and the first and fourth switching elements M1 and M4 are turned off. Therefore, through the third switching element M3, the power supply voltage Vdc supplied to the minus (−) terminal of the primary side of the transformer flows to the ground potential through the second switching element M2. Accordingly, the voltage Vpri in the primary side of the transformer maintains the negative voltage level, as shown in FIG. 2.
In the seventh interval {circle around (7)}, the first switching element M1 is continuously turned off because the first control signal S1 is continuously ‘logic high’, and the second switching element M2 is continuously turned on because the second control signal S2 is continuously ‘logic high’. The third switching element M3 is turned off because the third control signal S3 changes from ‘logic low’ to ‘logic high’, and the fourth switching element M4 is continuously turned off because the fourth control signal S4 is continuously ‘logic low’.
In the seventh interval {circle around (7)}, only the second switching element is turned on, and the first, third, and fourth switching elements M1, M3, and M4 are turned off. Therefore, when the third switching element M3 is turned off, the power supply voltage Vdc which has been supplied to the minus (−) terminal of the primary side of the transformer is not supplied any more through the third switching element M3. As a result, the voltage to be applied to the primary side of the transformer is 0 V, as shown in FIG. 2.
Finally, in the eighth interval {circle around (8)}, the first switching element M1 is continuously turned off because the first control signal S1 is continuously ‘logic high’, and the second switching element M2 is continuously turned on because the second control signal S2 is continuously ‘logic high’. Further, the third switching element M3 is continuously turned off because the third control signal S3 is continuously ‘logic high’, and the fourth switching element M4 is turned on because the fourth control signal S4 changes from ‘logic low’ to ‘logic high’.
Since the second and fourth switching elements M2 and M4 are turned on and the first and third switching elements M1 and M3 are turned off in the eighth interval {circle around (8)}, the voltage Vpri to be applied to the primary side of the transformer is maintained as 0 V, as shown in FIG. 2.
After that, the first to eighth intervals {circle around (1)} to {circle around (8)} are continuously repeated.
However, in the above-described method of driving a cool cathode fluorescent lamp inverter for backlight in which the lamp is driven by a sine wave, the lamp cannot be driven by the self discharge, even though a full-bridge inverter is used to control a voltage and current applied to the lamp in the case of the cool cathode fluorescent lamp having the internal electrodes.
In other words, only forming external electrodes causes a wall charge effect, where electric charges are accumulated on the glass wall of both electrodes when a voltage is applied, so that the lamp can emit light, which is referred to as ‘the self-discharge driving’. However, since the cool cathode fluorescent lamp has electrodes formed therein, the wall charge effect caused by forming external electrodes does not occur. Therefore, there is a problem that light cannot be emitted with high efficiency and at high luminance by the self discharge.
In addition, in the cool cathode fluorescent lamp, light is emitted only by the sine wave driving method, which makes it hard to drive a square wave voltage. As a result, the switching of the full-bridge inverter switching element cannot be controlled effectively.